I have posed this question to several people and get conflicting answers.

I am in Arizona and we have been having afternoon temperatures 115 and above. When that occurs, my heat pump compressor shuts off and does not cool.

Now a little history, in 2017, my ac was checked by a serviceman for the annual spring check up. He said I needed a new condenser fan motor because the old one has some rust on it. He replaced it with a generic motor with 825 RPMs while the old one was 850 RPMs.

Would the lower fan speed, though only 25 RPMs cause the compressor to shut down in the extreme heat of the day? The rest of the time, even in the winter heating time and with lower summer outside temperatures, the unit runs fine.

I am debating on getting a new 850 RPM motor if that will take care of my problem.

Thank You
Duane Smith
Mesa, AZ

There may be a thermal overload switch on the compressor. Make sure you have clean coils and unobstructed airflow. Don't think 25 RPM would make that much difference. 825 is a common speed. I think your ambient temperature is probably part of the issue.

The unit has worked fine for 10 years until the lower fan speed motor was installed, so that became suspicious to me. Thanks for your reply.

Did you have your 2018 check up and was everything fine then? The motor ran fine for a year no problems until THIS summer? Does the heat pump compressor motor function normally again after a certain amount of down time? You need an OHM meter to have your system PROPERLY diagnosed. Too many safety and run permissives to guess about. Capacitors and control boards will exhibit the same symptoms as a faulty thermostat or bad compressor motor, or as was said bad air flow due to dirty coils. All of them tends to shut things down.

Seems the regular check up guy would be the first call and easiest way to go at this point, if you suspect the motor and he installed it.

In order to keep my question simple, I didn't post the whole story. I only wanted to know if the 25 RPM slower motor would cause the unit to shut down in high outside temperatures. I realize other variables can also factor in. First of all, the unit is not mine and here is the whole story.


The unit I was referring to is a 3 ton American Standard-Trane heat pump, model WCH036C100AA, on the roof of a house. It belongs to a 92 year old neighbor of mine. I do all I can to help her stay in her house as long as she is able to get around. She does all her own yard work and takes care of her swimming pool, along with all her housework.


The problem started the spring of '17. She was in the 10th year of an annual checkup contract she had with a company that installed the unit. That company sold out to another company and this was the first time the new company was doing the checkup. The serviceman came off the roof and told here she needed a new condenser fan motor because there was rust on the old one. She didn't want the unit to fail since it was coming time for the hot weather. So, $900 later, she had a new motor and capacitor. But, when the weather did get hot enough that she needed cooling, the unit would not run.


This was when I first knew about the new motor and that she had called for the company to come out and check it. She wanted me to come down and be there when the serviceman was there. This was not the same one who installed the motor and he checked things out and told me the new capacitor was bad and he replaced it. He then spent 15 minutes or more telling us she needed to save up money for a new unit because hers is 10 years old and can fail at any time and that R-22 was going away and any repairs with it would be costly. He became a salesman instead of a serviceman.


The unit worked fine after that until the outside temperatures got up to 115 last summer so she called them out again. This time, the serviceman that installed the motor came out and he couldn't find anything wrong but told us, I was there again, it was because the unit was old and probably worn and wasn't working 100% and she should be looking at a new one. We kind of blew him off and he said he would go out to his truck and get an estimate for a new unit. He came back and said his office would get an estimate for her and either send it or call her. That was the last she heard from them, they never followed up.


The 110 and above temperatures continued for awhile and my brother that does handyman work, as I also do, does some light AC work went up on the roof one afternoon when it was really hot and checked pressures and all he could see was the serviceman had taken the sensor bulb away from the suction line, for what reason we could not figure out, so my brother reattached it. That didn't make any difference in the operation of the unit and the lady didn't want the company to come to her house ever again.


To get her by, my brother told her to run the unit at lower thermostat temperature to cool the house down until the hot afternoon and then shut it off for the hot hours. Then when it cooled down outside, the unit worked fine all night and until noon or so the next day. She said that worked fine. Then she wanted to see how it worked in the winter heating mode and again this spring. It was working fine until we got above the 110's last week. Actually, we had a spell about 3 weeks ago with the high temps but with low humidity and it made it through that fine. The higher humidity this week put more of a load on it and it started shutting down.


I suspected the AC company had put the wrong motor on the unit from the start of all of this. I looked at the receipt and it stated "generic truck stock 825 RPM motor". I figured out that this unit has a discontinued motor but I found there is a MOT12931 replacement that is 850 RPM for the OEM. I called Johnstone Supply, which has a storefront here and the counterman said they all they carry are generics and the 25 RPM would not make a difference. I also had posted to a forum and ask about that and the moderator came back and said it absolutely would make a difference.


So now, I am getting opposite answers and don't know what to do. My neighbor says if the higher RPM will take care of the problem, we may get a motor and I can put in on.


To make this story even longer, I did a search for reviews on the AC company and I was surprised to see almost all 35 or more reviews were 5 stars with only a couple of negatives. The owner responded to those and even gave his cellphone number to them and said he would try to work out the problems. The reason I haven't called him is because I figured that he would say 25 RPM difference would not cause our problem and I am trying to verify whether it would and he will just want to CYA.


I feel that any AC unit has a break point temperature where they will stop working and they are made to just get to that point with not much leeway and if anything is not functioning 100%, it will not even get to that point. If a little dirt can affect the air flow and shut the unit down, it would make sense that any reduction of fan speed would also. I also think American Standard put an 850 RPM fan motor in this unit for a reason.

Thanks for reading this and for your replies.
Smitty

https://www.repairclinic.com/RepairH...er-not-cooling

Review all the causes. It's hard to diagnose on line just from symptoms since they all have to be checked, but personally I see no real difference between 850 and 825 since 900 RPM's is 900 RPMS's. You need a second opinion, and if a second opinion says it's the motor then the first guys is on the hook since he supplied it. It's a pretty simple thing to check, rather than go round and round on. All you need is an ohmmeter. Trying to find you some videos to check and confirm the motor gets a signal and keeps it for that AC phase.

Actually guy you should get 3 estimates for both repair or replace, which should be free, or the service charge applied to the repair. A few phone calls will get that information. Personally I have a problem with any contractor than suggests a new unit without telling you specifically what the repair is for the old one. Interacting with other companies also gives you a better price range than just the first one so I can appreciate you wanting to know if he was telling the truth, or blowing smoke for a sale.

Sorry that's the best suggestion I have given the situation.

Sounds like you have done everything except change the contactors or check the thermostat. One bad wire will do the same as a bad motor. Don't be stuck on one thing when it could be many.